1. Field of the Invention
The present invention relates to a monitoring apparatus for a centrifuge instrument that monitors the energy applied to the instrument to accelerate a rotor mounted therein.
2. Description of the Prior Art
A centrifuge instrument is a device by which liquid samples may be subjected to a centrifugal force field. The sample is carried within a member known as a centrifuge rotor. The rotor is mounted at the top of a rotatable drive shaft that is connected to a source of motive energy.
The centrifuge instrument may accept any one of a plurality of different centrifuge rotors depending upon the separation protocol being performed. Whatever rotor is being used, however, it is important to insure that the rotor does not attain an energy level which exceeds the capacity of the energy containment system of the instrument.
The energy containment system includes all structural features of the centrifuge instrument which cooperate to confine within the instrument any fragments produced in the event of a rotor failure. These structural features include, for example, one (or more, concentric) guard ring(s), instrument chamber door and associated door latches. The energy containment system, however configured, has a predetermined energy containment threshold.
The total energy input to a system is equal to the sum of the energy dissipated in operation and the stored energy. In a centrifuge instrument the dissipated energy is that portion of the applied energy that is needed to overcome the inherent losses due to the mechanical drive system or due to fluid friction. This portion of the applied energy is dissipated as heat. The remaining portion of the applied energy is stored by the motion of the rotor. If the stored energy of a failed rotor exceeds the energy containment threshold of the instrument a fragment of the rotor may not be confined by the containment system, but may instead exit therefrom. Any fragment which exits the instrument presents an extremely serious threat of injury and/or damage. It is the stored energy that must thus be contained in the event of rotor failure.
The stored energy of motion, or the kinetic energy, of a rotor is directly related to its angular velocity, as specified by the relationship: EQU Kinetic Energy=1/2(I .omega..sup.2) (1)
where I is the moment of inertia of the rotor, and PA1 where .omega. is its angular velocity.
Presently, the most direct manner of limiting rotor energy is to limit the velocity (i.e., the angular velocity), or the speed, that the rotor is able to attain.
One manner of rotor speed limitation is achieved by windage limiting the rotor. Windage limitation is a passive speed limitation technique. Windage limitation is achieved by purposely designing the rotor is a way that any excess energy above that level necessary to overcome frictional losses in the rotor drive system and to drive the rotor to predetermined safe speed is dissipated as windage, or air friction.
Another way to limit rotor speed is to provide an overspeed control system in the instrument that affirmatively, or actively, limits the speed at which each given rotor is allowed to spin. For an active overspeed control system to limit rotor speed effectively it is necessary to ascertain the identity of the rotor mounted in the instrument.
Rotor identity information may be directly derived from the operator by requiring that the operator input identity information to the control system prior to the initiation of a centrifugation run. However, to protect against the possibility of an operator mistake, automatic rotor identity arrangements are used. These rotor identity arrangements automatically identify the rotor present on the drive shaft of the instrument and, based on this identification, permit only that energy to be applied to the rotor to permit it to reach a predetermined allowable speed.
Various forms of automatic rotor identity arrangements are known. In one form each rotor in a rotor family carries a speed decal having bands or sectors of differing light reflectivity. The pattern on the decal contains a code to establish rotor identity. The code is read by an associated sensor at a predetermined low angular velocity. U.S. Pat. No. 4,205,261 (Franklin) is representative of this form of rotor identity arrangement. In another form each rotor in the family carries a predetermined pattern of magnets. The magnets are sensed by a suitable detector, typically a Hall Effect device, to read the rotor code. U.S. Pat. No. 4,601,696 (Kamm) is representative of this form of rotor identity arrangement.
Other forms of automatic rotor identity arrangements sense a particular parameter of rotor construction in order to identify the rotor. In the arrangement disclosed in U.S. Pat. No. 5,037,371 (Romanauskas), assigned to the assignee of the present invention, the shape of a rotor mounted on the drive shaft is interrogated ultrasonically to generate a signal representative of the rotor's identity. In U.S. Pat. No. 4,827,197 (Giebeler) the inertia of the rotor mounted on the shaft is detected and used a the basis of a rotor identity signal.
Because each of the above-discussed forms of automatic rotor identity arrangement is focused toward the use of secondary, rotor-based characteristics, an additional layer of complexity is added to the rotor speed control scheme beyond a basic speed control determination. Accordingly, for the sake of simplicity, it is believed advantageous to provide an instrument control system that uses available basic, readily ascertainable information associated with instrument operation to limit energy applied to the rotor and thereby to prevent the stored energy of the rotor from reaching a value that challenges the energy threshold of the energy containment system of the instrument.